This invention relates to computer or otherwise automatically driven photoexposure devices for exposing lines and symbols on a photosensitive surface to produce artwork for printed circuits, photomasks and the like; and deals more particularly with a circuit for flashing the lamp of a photoexposure device of the type which generates lines by repetitively projecting light spots onto the surface at a relatively high rate while the locus of the spots is moved relative to the surface.
In photoexposure devices of the type with which this invention is concerned, that is, those which create lines by flashing spots of light, to obtain a high writing or line generating speed it is necessary, for high quality artwork, that the device be able to produce the spots at a high repetition or flash rate with excellent control over the amount of light contained in each flash.
The lamp used in the circuit of this invention is a Xenon lamp or a similar gas filled lamp. Such a lamp is commonly flashed by a circuit which includes a triggering capacitor and a firing capacitor. Discharging the triggering capacitor through an associated circuit produces a high voltage pulse across the lamp which ionizes a conducting path from its anode to its cathode. The main light producing conduction, energized by discharge of the firing capacitor, then follows this conducting path at a higher current and a lower voltage than that of the high voltage pulse which established the ionized path. Repeated firing of the lamp at high rates, however, gives rise to certain highly complex phenomena within the lamp. The net result of these is that the arc tends to have variable length in the lamp depending on the history of previous arc paths. The heated gases within the lamp tend to circulate providing nonuniform breakdown characteristics in the gas. In addition, impingement of ions or electrons at various portions of the internal metal electrodes tends to condition them to be better or poorer sources for the present discharge depending on past history. Because of these factors, the voltage at which the arc extinguishes may vary in accordance with the previous internal history of the lamp. Therefore, if the firing capacitor is charged to the same level during each flash cycle different amounts of light may be contained in different flashes due to the arc extinguishing at different voltages and thus having different durations.
The general object of this invention, is, accordingly, to provide an electronic circuit capable of flashing a flash lamp at a very high repetition or flash rate (flashes per second) and with a substantially constant output of light per flash.
A more particular object of the invention is to provide a lamp flashing circuit of the foregoing character wherein the rate of flashing is optimized by, among other things, initiating a discharge of the triggering and firing capacitors immediately, or within a few microseconds, of their reaching their desired charge voltages.
Another object of the invention is to provide a lamp flashing circuit of the foregoing character wherein uniform light output per flash is obtained by initiating the discharge of the triggering and firing capacitors by detecting the triggering capacitor voltage and initiating discharge when such voltage reaches a given reference value, whereby the voltage achieved by the firing capacitor during charging will depend on the voltage remaining thereon upon extinguishment of the arc in the previous cycle, thereby automatically compensating for changes in the voltage level at which arc extinguishment occurs.
Other objects and advantages of the invention will be apparent from the drawings and from the following description.